The only planet
ever predicted by mathematical calculations, Neptune was discovered
by Johann Gallee in 1846 and is invisible to the unaided eye thanks
to its shining at an apparent magnitude of around +8. Uranus takes
about 84 years to orbit the Sun and thus spends over 13 years in any
given constellation of the zodiac.

Thanks to its immense
distance, even in telescopes, the only way to tell Neptune apart from
background stars is to observe it at high powers and look for the
distinctive, clean-cut planetary disc.